Projectors such as a liquid crystal projector have been known. In these projectors, light is irradiated onto the back side of light valves, such as in a liquid crystal display panel array, and the light is then modulated by picture information input to these light valves. The transmitted light is then projected by a projection lens onto a screen as an enlarged image.
As projectors are used in various places with various size audiences, it is advantageous if the focal length of the projection lens can be varied so as to select a projection distance that matches a particular size of projection screen. For example, a projector may be purchased with the intent of using it in a rather small room with a nearby projection screen, but a necessity for using the projector in a large meeting room with a larger projection screen can also arise. In such a case, if an attachment lens for extending the focal distance is added to the projection lens, the projector can be conveniently positioned on the side of the room opposite the screen, with the audience seated between the projector and the screen. Such an attachment lens is called a teleconverter lens because the focal distance is converted towards the telescopic side.
For example, a teleconverter lens as described in Japanese Laid Open Patent Application H10-197792 has been known. This teleconverter lens comprises a front lens group at the large conjugate length side (hereinafter termed the enlarging side) of the teleconverter lens, and a back lens group at the small conjugate length side (hereinafter termed the reducing side) of the teleconverter lens, with a wide air space between these two lens groups. The front lens group is composed of from two to three lens elements, and the back lens group is composed of two lens elements. Further, a combined lens (e.g., one formed of positive and negative lens elements that are cemented together) is arranged on either side, or somewhere within, the teleconverter lens in order to correct for achromatism.
However, the teleconverter lens described above was primarily developed for use as an image pick-up lens of cameras. Thus, it is unsuitable for projecting very fine picture elements of a projection system, especially those that use a liquid crystal panel to modulate the image light onto a screen. A light beam which is transmitted through the liquid crystal display panel and is incident into this projection lens passes successively through the negative focusing lens group at the enlarging side of the projection lens and through the negative back lens group of the teleconverter lens. Thus, certain aberrations generated by the focusing lens group of the projection lens are not compensated for, but are instead enlarged, in passing through the back lens group of the teleconverter lens. More particularly, aberrations such as distortion and lateral color that are introduced by the projection lens are increased by the teleconverter lens to such an extent that the projected images are unsatisfactory.
This invention relates to a teleconverter lens, and particularly to a teleconverter lens that may be mounted to the enlarging side of a projection lens for increasing the focal length of the projection lens. The object of the present invention is to provide a teleconverter lens which may be mounted to a projection lens for increasing its focal length without degrading the image quality of images projected by the projection lens.